1. Field of the Invention
This invention relates in general to systems and methods for driving large capacity AC motors and, in particular, to such systems and methods including a transformer.
2. Brief Description of Related Art
The oil drilling industry uses large volume submersible pumps typically located thousands of feet into a well. The pump assembly includes a centrifugal pump which is driven by an AC motor. The motors may range from 15 to 2000 horsepower, and thus, require a large supply of power. Normally, 60-cycle, three-phase power is supplied with voltage phase-to-phase being 480 volts or more. Common rotational speeds of the motor are about 3500 revolutions per minute (rpm). Most of these types of pumps are generally single speed pumps. Because of different viscosities, densities, well flowing characteristics, and the like, it is desirable to vary the speed of the motor. One way in which to vary the speed is to vary the frequency of the power being supplied. Normally, however, the line power comes from a utility company and cannot be changed from the standard 60-cycle per second. There are circuits that will convert the standard frequency to different frequencies. These circuits can also change the amplitude in proportion to the frequency change for efficient operation of the motor.
A technique for controlling the speed of a three-phase induction motor uses an electronic variable frequency drive (VFD). The electronic VFD has a rectifier circuit that requires multiple phases of alternating current. For example, a six-pulse rectifier needs three-phases of electric power to be input so that six pulses are provided by the full-wave rectification. One type of VFD uses pulse width modulation (PWM). Others use square waves, such as a six-step waveform.
Although multi-phase PWM inverters are useful, they can cause high order harmonic voltages resulting in detrimental high peak voltages at the motor, particularly with transformers and long lengths of cable between the VFD and the motor.
System filters consisting of an inductor and a shunt capacitor can be used to prevent or attenuate harmonic distortion. Applicant has recognized, however, that these filters have only been marginally successful due to problems related to environmental exposure. Applicant has also recognized that positioning the inductors in containers sufficient to prevent degradation due to the environment is problematic due to the excessive heat generated in the inductor coil of the filter arrangement.